1. Field of the Invention
Embodiments of the present invention are directed to an optical element having both an amplitude diffractive structure and a phase diffractive structure and/or a vortex lens.
2. Description of Related Art
It is very difficult to manufacture a multi-mode fiber with good control over the index of refraction in the center of the fiber. If the light coupled to the fiber excites some modes that propagate mostly in the center of the fiber and other modes which do not propagate mostly in the center of the fiber, very different propagation times for these modes may result. This is referred to as differential mode delay. Differential mode delay tends to spread out the pulse length of signals and reduce the effective bandwidth of the fiber.
Modes which propagate mostly in the center of the fiber are the lower order fiber modes, i.e., modes having small propagation angles that strike at or near the center of the fiber. These lower order modes spend most of the time in the center of the fiber, tend to travel straight down the fiber and the shape of these modes does not change much as they propagate. Therefore, in order to reduce differential mode delay, any light which enters near the center of the fiber needs to be incident at an angle which is large enough not to excite lower order modes, but not so large that the critical angle is exceeded and the light fails to be coupled or no light should be input to the center of the fiber.
One current solution involves coupling light into single mode fibers which are then positioned off-axis relative to the multi-mode fiber. Single mode fibers have a much smaller core than multi-mode fibers, so can be used to provide light at specific positions on the endface of the multi-mode fiber. However, single mode coupling is more expensive than multi-mode coupling and the additional coupling step leads to an increase loss in light. Further, while no light enters the fiber of the center for this configuration, the light will still cross the fiber axis as it propagates, thus increasing the differential mode delay. Additionally, ferrules or other structures housing the multi-mode fiber to a single mode fiber junction are not readily available and must be developed specifically for that purpose.
Another solution is to use a vertical cavity surface emitting laser (VCSEL) excited to radiate in a ring mode. The operation of the VCSEL in radiation modes other than the lowest order have less predictable flux distributions than in the lowest order mode, in which the distribution more closely approximates a Gaussian profile. Further, there will still be some power in the lower order modes of the VCSEL. Additionally, such operation of the VCSEL often requires a higher current to drive the source into the higher radiation modes.
As the use of non-physical contact connections between light sources and fibers increases, the need for effective isolation to prevent light reflected at the fiber interface from being returned to the light source increases. Feedback to the light source may result in spectral broadening, light source instability, and relative intensity noise, which affect the monochromaticity of the light source. As data rates go up, the systems become more sensitive to relative intensity noise and require low bit error rates. Conventional optical isolators using polarization effects to attenuate reflection are very expensive, making the non-physical contact impractical. The importance of avoiding feedback is further increased when trying to use cheaper light sources, such as vertical cavity surfaces emitting laser diodes and light emitting diodes.
One solution that avoids the use of an optical isolator is a mode scrambler that divides power from the light source into many modes. A configuration employing a mode scrambler includes a single mode pigtail that provides light from the light source to the mode scrambler that then delivers the light to a transmission cable via an air-gap connector. Since any reflected power will still be divided across the many modes, any reflected power in the mode that can efficiently be coupled into the pigtail is only a small fraction of the total reflected power, thereby reducing return losses. However, this solution involves aligning another fiber, physically contacting the fiber with the mode scrambler, and placing the light source against the fiber. This pigtailing is expensive. Thus, there still exists a need for true non-physical contact connection between a light source and a transmission system that does not require an isolator.